"Hi Mike, I was able to clone 10.8 from my MacBook Pro to a spare SATA hard drive in my Mac Pro 1,1. I have been booting the Mac Pro in 64-bit mode using the Chameleon -r1921 boot loader for several weeks, anticipating the release of Mountain Lion. After editing the supported machines plist and tweaking the installed extensions a little 10.8 seems to be running just fine.
(Not sure if it works for 10.8 (i.e. if it requires the file present), but for OS X 10.7 installs most older Mini owners w/C2D upgrades just deleted the supported models plist, although some did edit it for their model ID. (Either way, sometimes an OS update (combo) may restore/rewrite the plist file so keep a backup if you've edited one.)-Mike)
I didn't try deleting the .plist, I just edited it and I did save a copy in my documents folder in case future updates rewrite it.
Most of my help on this came from the two pages you referenced (repeated below), they are excellent resources.

I have a lot invested in this Mac- 2 SSDs, 16 GB RAM, ATI 6870- so I wasn't happy that Apple dropped support of the 1,1 but "unsupported" just means it takes a little extra work.

(he later wrote)I ran a couple of tests late last night, Rember shows all 16 GB of my RAM working properly (some 1,1 owners have reported RAM problems going to 64-bit) and Geekbench scored 6974, up from 6180 in Lion 32-bit with the same hardware.

(8/25/2012 Update/Note on OS X 10.8.1 update OK)
I sent a note a few weeks ago about running Mountain Lion on a MacPro 1,1 (booting 64-bit via Chameleon).
I updated to 10.8.1 with no trouble, I didn't need to redo the edit on the supported machines plist.
-Dave"